Urea is the most common nitrogen-containing fertilizer. Urea has the highest nitrogen content of all nitrogen-containing fertilizers in common use (46%). Its consumption worldwide has been considerably increased, from about 20 million tons in the early seventies to about 100 million tons at the beginning of the twenty first century. Nitrogen is the basic constituent for any living system as a constituent of protein.
Due to intensive farming and the reduction of sulphur emissions in the air by industry and the subsequent supply to the ground via rain, modern agriculture requires sulphur in addition to nitrogen.
Good agricultural practice usually require nitrogen and sulphur in a ratio 10/1 to 5/1 in order to answer to the crop demand, for example 150 kg nitrogen/ha/year and 30 kg sulphur/ha/year.
Lack of sulphur results both in a lower quantity and a lower quality of crops, and sulphur deficiency is often reflected in the content and type of proteins. Sulphur is indeed a major element entering into the chemistry of the cells in molecules such as amino acids (cysteine, methionine, etc.). It is also a catalyst for the photosynthesis and, in some cases, may improve the fixation of atmospheric nitrogen.
Conventionally, sulphur has been applied to the soil in the form of elemental sulphur, or as compounds such as ammonium sulphate, ammonium bisulphate, thiosulfates, sulphides or gypsum, or in combination with other fertilizer materials such as urea, for example as a physical blend of urea and ammonium sulphate, or as a co-granulated urea and ammonium sulphate material, the latter which is hereinafter called urea ammonium sulphate, abbreviated as UAS.
Like urea, the urea that is present in UAS is also hydrolysed in the soil under the action of an enzyme catalyst, commonly called urease, to produce ammonia and carbon dioxide. Ureases are found in numerous bacteria, fungi, algae, plants and some invertebrates, as well as in soils, as a soil enzyme. Urease activity tends to increase the pH of its environment as the ammonia is dissolved into the water in the soil, which is taken up by the plants as their principle source of nitrogen. However, ammonia can also be released into the atmosphere, thus becoming unavailable for the plant root system, a process called ammonia volatilization. Up to 50 weight % of nitrogen can be lost as a result of the volatilization of ammonia, all depending on the soil type, water content, pH, climate conditions, etc.
The availability of nitrogen, originating from urea, to the root system of plants can be improved by combining (i.e. by incorporation or addition) a urease inhibitor with a urea-containing fertilizer. Urease inhibitors are compounds that are capable of temporarily reducing the activity of the enzyme and slow down the rate at which urea is hydrolysed. There are many compounds that can inhibit urease, but only a few that are non-toxic, effective at low concentrations, chemically stable enough and able to be combined with urea-containing fertilizers.
Among the most effective urease inhibitors known today are the phosphoric triamide compounds, first disclosed in U.S. Pat. No. 4,530,714 (Allied Corporation, 1985).
An example of an effective urease inhibitor, disclosed in said patent is N-(n-butyl) thiophosphoric triamide, which will be referred to herein as nBTPT. This compound is actually the precursor for the active compound N-(n-butyl) phosphoric triamide (nBPT), obtained through oxidation of the thio-compound, but it is the thio-compound that is commonly produced, sold and used. Throughout this application, when referring to urease inhibitors of the type phosphoric triamide, it is understood that this comprises all active compounds, active precursors and active conversion products, resulting from said phosphoric triamides.
When combined with a urea-containing fertilizer, phosphoric triamide compounds reduce the rate at which urea is hydrolysed to ammonia in the soil. The benefits that are realized as a result of the delayed urea hydrolysis include the following: (1) nutrient nitrogen is available to the plant over a longer period of time, (2) excessive build-up of ammonia in the soil following the application of the urea-containing fertilizer is avoided, (3) the potential for nitrogen loss through ammonia volatilization is reduced, (4) the potential for damage by high levels of ammonia to seedlings and young plants is reduced, (5) plant uptake of nitrogen is increased, and (6) an increase in crop yields is attained. While phosphoric triamide compounds do not directly influence the rate of ammonium nitrification, they do control the levels of ammonium which are subject to the nitrification process and thereby indirectly controls the levels of nitrate nitrogen in the soil.
Surprisingly, the inventors have now been confronted with the problem that the urease inhibitor of the type phosphoric triamide, when applied as a liquid, which is the most common commercially available form, is not stable when in contact with urea ammonium sulphate material, despite the fact that the use of a urease inhibitor of the type phosphoric triamide is generally disclosed for urea-based fertilizers, such as UAS. Moreover, even a stabilised urease inhibitor of the type phosphoric triamide in an alkaline organic solvent, such as a mixture of propylene glycol and N-methylpyrrolidine is rapidly degraded. In this context, with stability is meant that the original urease inhibitor is chemically degraded. This finding was most surprising, but stimulated the inventors to find a solution for the problem of stabilizing the urease inhibitor (i.e. reducing its degradation) in the presence of urea ammonium sulphate material.